Definition of best friend...

Don't worry i will not leave you!

Pretty much agree with that definition.
A best friendship develops slowly; you can never know who they might become when you first meet them, but there is a resonance, a meeting of minds, shared values and interests, and a strong feeling of equality. There's an instant feeling of liking and trusting, with no hint of background discomfort or doubt.
Then it develops from there, from the sheer joy of spending time in each other's company, doing different things together, opening and sharing experiences and feelings.
If they live nearby, they do soon become like members of the family. There's a willingness to forgive foibles and mistakes. There's a commitment to mutual honesty and resolving differences.

The only point I disagree on is the one about a friend being a guide. I see advice as only offered when asked.
A friend may indeed have a more objective outside view about one's ways and choices, but she lets one live one's own way, and then is there to help if anything goes wrong, and vice versa.

brokenheartforever i think them sort of friends are rare as gold dust and always so good to have around and be with

To me friends are the ones just waiting to stick a knife in my back